1. Technical Field of the Invention
The present invention relates in general to the field of analog-to-digital converters (ADCs), and in particular by way of example but not limitation, to digital calibration of ADCs in which the calibration may be accomplished adaptively with dynamic estimation of reference signals that have unknown parameters.
2. Description of Related Art
The natural world operates in an analog domain, but information signals (voice, data, etc.) may frequently be processed, transmitted, or otherwise manipulated more efficiently in the digital domain. The conversion from the analog domain to the digital domain is accomplished with ADCs. An ADC receives as input an analog signal and produces as output a digital signal. However, some information present in the analog signal is necessarily lost during the conversion process even if an ADC is operating in an ideal manner. Unfortunately, real-world ADCs do not operate in an ideal manner. Consequently, the digital output of a real-world ADC does not track the analog input even as accurately an ideal ADC.
It is therefore beneficial to make and/or tune real-world ADCs to approximate ideal ADCs. Techniques have been developed to calibrate real-world ADCs so as to modify their performance to emulate ideal ADCs as closely as possible. For example, ADCs are traditionally calibrated using high precision digital voltmeters to characterize the errors that result from digitizing static or slowly varying analog reference voltages. The outcome from this static testing forms the basis for a hardware or software implemented calibration scheme. Another method of conventional ADC calibration is the use of a sinusoidal reference signal. The reference is sampled, and estimations of the ideal sample values are calculated. These estimations are calculated using a minimum squared error criterion that requires knowledge of the frequency of the calibration signal. The errors (i.e., the difference between the estimated values and the actual sampled values output by the ADC being calibrated) are then used to build a correction table. The correction table may subsequently be used to modify sampled values of actual (e.g., non-calibration, functional, etc.) analog input signals.
Efficient calibration schemes require that the reference signal be dynamically estimated on a sample-by-sample basis during the ADC calibration period(s). No method previously existed for dynamic estimation of a reference signal (e.g., a calibration signal) with one or more unknown parameters (e.g., frequency, phase, etc.) during an ADC calibration. The pre-existing calibration procedures relied on accurate and costly signal generators and/or precise and expensive measuring components.
However, the parent application (U.S. Ser. No. 09/196,811, now U.S. Pat. No. 6,127,955) addressed these deficiencies of pre-existing calibration procedures by dynamically estimating a reference signal having one or more unknown parameters. Nevertheless, the invention of the parent application was primarily directed, with respect to frequency estimation, to the problem of calibrating ADCs that operate in a narrow frequency band. Since the linearity errors in general are frequency dependent, correction tables in accordance with the invention of the parent application are primarily useful for frequencies near the calibration frequency. The invention of the parent application does not therefore address the problem of wide-band calibration of ADCs. Consequently, implementations in accordance with the invention of the parent application do not optimally calibrate ADCs that are to be operated in a broad frequency band.
The deficiencies of the prior art are overcome by the method, arrangement, and system of the present invention. To wit, the present invention is directed to a method, arrangement, and system for enabling adaptive calibration of an analog-to-digital converter (ADC) from reference signals with unknown parameter(s). For example, a reference signal having at least one unknown parameter that varies may be advantageously employed in order to calibrate an ADC over wide ranges of the unknown parameter(s) (e.g., over a wide frequency range).
In certain embodiments, an analog reference signal s(t) is supplied to an ADC to be calibrated. The output x(n) of the ADC is used by a, e.g., sine-wave reconstruction filter to reconstruct an estimate ŝ(n) of the sampled instances of the signal s(t). A recursive frequency estimator within the sine-wave reconstruction filter produces a frequency estimate of the analog reference signal s(t) from the signal x(n) . An adaptive reconstruction filter uses this frequency estimate and the signal x(n) to produce the estimate ŝ(n). When a convergence detector determines that the adaptive reconstruction filter has converged, the estimate ŝ(n) is used to alter values that are stored in a correction table. During functional operation of the ADC, the correction table is used to correct the output values of the ADC.
The above-described and other features of the present invention are explained in detail hereinafter with reference to the illustrative examples shown in the accompanying drawings. Those skilled in the art will appreciate that the described embodiments are provided for purposes of illustration and understanding and that numerous equivalent embodiments are contemplated herein.